A team of forensic researchers from the Metropolitan Police in London, UK, claim to be able to accurately timestamp any audio recording—using just the background electrical hum present in any digital recording.

The grid supplies power across the country in the form of AC electricity. The signal that's pumped across the country and into our homes has a main frequency at which it oscillates—60 Hz in North America or 50 Hz in Europe. The presence of mains power creates low-level interference, at that frequency, that all our electronic devices have to deal with. Its perceptible in audio recordings as a quiet background hum.

For most of us, that noise is irritating. But the forensic researchers have observed that the frequency of the hum actually changes subtly over time, with fluctuations of a few thousandths of a hertz. That's a result of supply and demand: when loads are higher, the frequency drops just a little.

Interestingly, the way these changes happen is unique—which means that the variation in that irritating hum can be used as a fingerprint. Compare variations of frequency around the 60 Hz mark in an audio recoridng to a log of national variations, and it's possible to accurately pinpoint the date and time the recording was made with no other information.

That's why the researchers at the Metropolitan Police have been logging such information 24 hours a day, 365 days a year for the past seven years. In fact, the technique has already been used in a court case against a team selling weapons illegally—and the police came out on top.

Of course, the technique is easier to use in the UK because a single grid provides electricity for the whole country. In the US things aren't quite so simple—but by logging variations of supply frequency on each grid, the same results should be possible. [BBC]